Molar-tooth structures generally form a network of interconnected vertical and horizontal calcite ribbons and occasional spheroidal objects or “blobs” found in fine-grained, probably marine sediments spanning the late Archean to late Neoproterozoic interval, a duration of ~1900 m.y., or nearly half Earth's stratigraphic record. Vertical ribbons, averaging 5 mm in thickness, are generally intricately folded or fragmented by compaction. Decompaction shows that some ribbons may have been up to a meter in length perpendicular to bedding, forming in sediments that originally contained ~70% water, implying formation in sediment depths of ~1.7 m or less. Though at least 10 processes have been suggested for the blobs and ribbons, they were most likely voids created by the rise of gas bubbles from the decay of microbial mats that were rapidly filled with calcite. Horizontal ribbons probably formed under conditions of high pore-fluid pressure due to an overlying seal attributed to extracellular polymeric substances (EPSs) associated with the microbial mats. Cathodoluminescence imaging reveals that several molar-tooth fills show two distinct components: (1) earlier granular cores (now calcite) that are overgrown by (2) polygonal calcite. The earlier cores may have begun as an amorphous calcium carbonate (ACC) phase that recrystallized as calcite. Molar-tooth structures appear to simply reflect the formation of compacted calcite-filled voids under a seal of microbial mats and EPSs. Molar-tooth structures were present on Earth and may have played a significant role in the earlier history of other rocky extraterrestrial bodies with aqueous environments.

Abstract The Sr and C isotope compositions of Neoproterozoic carbonates have been argued to provide important constraints on the timing and origin of depositional events, including ice ages. We investigated the geochemical signatures of immediately preglacial Sr-rich limestones from an unusually complete section in E Greenland and compared with limestones in an analogous position in the basin stratigraphy from N China. In some Greenland sections, slope facies grade upwards into glacimarine deposits, whereas in others slope facies are overlain by platform carbonates, then terrestrial eolian or glacial deposits without signs of erosion. The slope facies are mixed carbonate (ferroan dolomite and siderite)-siliciclastic (berthierine-quartz) shales with local limestone beds, variably disrupted by synsedimentary slumping. The shelf facies consists of mostly massive limestones with local molar-tooth structure. High Sr contents and poor fabric preservation indicate a primary mineralogy dominated by aragonite. Carbon isotopic values are negative in the slope facies whereas the overlying platform facies are isotopically positive, with mean δ 13 C values rising from +4.3 to +7.5‰ upwards. The current paradigm is that such a negative-to-positive anomaly reflects the termination of a glacial event elsewhere. We prefer the alternative Uiat deposition occurred across a stratified narrow ocean basin in which δ 13 C decreases significantly at depth. Samples from the platform facies with over 2000 ppm Sr and negligible Mn have 87 Sr/ 86 Sr ratios around 0.70635. These are rather lower than previously reported values in preglacial platformal sediments in the basin, and consistent with no increase in continental erosion prior to glacial deposition. The Dalian area of the North China block contains a 4-km-thick, tectonically deformed Neoproterozoic shallow marine succession, at the top of which are 80 m of storm-dominated limestone facies with abundant molar-tooth structures passing up into thin peritidal and dolomitic units below an erosion surface thought to correlate with glaciation elsewhere. Sr isotope ratios correlate with Mn, Fe, and insoluble residue and inversely with Sr, indicating the diagenetic addition of heavy Sr by interaction with silicate phases. Apparently unaltered signals are found in samples with Sr > 500 ppm and Mn < 100 ppm, with 87 Sr/ 86 Sr ratios decreasing upward from 0.7069 to 0.7063 from 70 m to 20 m below the top of the limestones. Nearly all δ 13 C analyses lie between +1.7 and +3.3‰. These limestones correlate with preglacial carbonate-platform facies in the E Greenland-NE Svalbard basin, but cannot be tied to a specific horizon. Whereas a Mn/Sr ratio of <2 is commonly taken to be a sufficient screening criterion for Sr Proterozoic isotope stratigraphy, increases in 87 Sr/ 86 Sr were found at Mn/Sr ratios as low as 0.2 in both regions. Availability of suitable lithologies is currently a major limitation on Sr isotope chemostratigraphic studies. However, Sr-rich Neoproterozoic limestones may achieve a 87 Sr/ 86 Sr resolution of (±0.0002. Deposition of such limestones bearing molar-tooth-type cracks ceased at around the time of a glaciation traditionally regarded as Varangian (c. 600 Ma), but which could conceivably be Sturtian (c. 740 Ma).

Abstract In the Precambrian world, devoid of higher organisms except near its end, carbonate sediments formed by a variety of abiotic and microbial processes, with patterns of deposition determined by tectonic, eustatic, and climatic processes. These ancient rocks demonstrate that the fundamental tenets of carbonate production and accumulation were initiated early in earth history, with the basic attributes of carbonate sedimentation well established by Neoproterozoic time. The broad temporal patterns of Precambrian carbonate facies composition and disposition parallel the long-term evolution of the earth’s oceans and atmosphere. Archean and Paleoproterozoic carbonates commonly contain abundant sea-floor precipitates, whereas the Neoproterozoic record is dominated by clastic-textured facies and abundant carbonate mudstones; Mesoproterozoic carbonates are transitional. Mesoproterozoic and early Neoproterozoic carbonates also contain abundant quantitites of the enigmatic molar-tooth structure. Grainstones, dominated by giant ooids with centimeter-scale diameters, are characteristic of many Neoproterozoic carbonates. Texturally unusual carbonates, featuring a reprise of Archean-style sea-floor precipitates, often cap glacial deposits of middle Neoproterozoic age. The influence of biology on sediment texture is best expressed in the history of Precambrian reefs. Archean through Mesoproterozoic reefs are dominantly stromatolite-based. Lamination textures reveal the progressive shift from in situ precipitation of aragonite and calcite encrusting the sea floor in Archean through Paleoproterozoic stromatolites to textures consistent with accretion of loose sediment through trapping and binding in Neoproterozoic stromatolites. This trend is interpreted to reflect the progressive decrease of abiotic factors and the concomittant increase of benthic microbial mats on controlling stromatolite growth. Neoproterozoic reefs witness the appearance of more complex textures that likely involve the participation of calcified microbes and noncalcified higher algae in colonizing the seafloor, increasing its surface complexity and resulting in highly porous frameworks for the first time in geologic history. Terminal Proterozoic thrombolitic reefs additionally contain the first calcified metazoans.

Abstract In the Precambrian world, devoid of higher organisms except near its end, carbonate sediments formed by a variety of abiotic and microbial processes, with patterns of deposition determined by tectonic, eustatic, and climatic processes. These ancient rocks demonstrate that the fundamental tenets of carbonate production and accumulation were initiated early in earth history, with the basic attributes of carbonate sedimentation well established by Neoproterozoic time. The broad temporal patterns of Precambrian carbonate facies composition and disposition parallel the long-term evolution of the earth’s oceans and atmosphere. Archean and Paleoproterozoic carbonates commonly contain abundant sea-floor precipitates, whereas the Neoproterozoic record is dominated by clastic-textured facies and abundant carbonate mudstones; Mesoproterozoic carbonates are transitional. Mesoproterozoic and early Neoproterozoic carbonates also contain abundant quantitites of the enigmatic molar-tooth structure. Grainstones, dominated by giant ooids with centimeter-scale diameters, are characteristic of many Neoproterozoic carbonates. Texturally unusual carbonates, featuring a reprise of Archean-style sea-floor precipitates, often cap glacial deposits of middle Neoproterozoic age. The influence of biology on sediment texture is best expressed in the history of Precambrian reefs. Archean through Mesoproterozoic reefs are dominantly stromatolite-based. Lamination textures reveal the progressive shift from in situ precipitation of aragonite and calcite encrusting the sea floor in Archean through Paleoproterozoic stromatolites to textures consistent with accretion of loose sediment through trapping and binding in Neoproterozoic stromatolites. This trend is interpreted to reflect the progressive decrease of abiotic factors and the concomittant increase of benthic microbial mats on controlling stromatolite growth. Neoproterozoic reefs witness the appearance of more complex textures that likely involve the participation of calcified microbes and noncalcified higher algae in colonizing the seafloor, increasing its surface complexity and resulting in highly porous frameworks for the first time in geologic history. Terminal Proterozoic thrombolitic reefs additionally contain the first calcified metazoans.